Technical Field
The present disclosure relates to measuring an unknown resistance using a reference resistance of a known value.
The measuring of unknown resistances can be applied in the field of sensor electronic interfaces, for example for automotive applications.
Description of the Related Art
The disclosure relates to electronics that manage sensors of a resistive type, for example for the automotive field. The operating principle of these sensors is based upon the variation of an electrical resistance as the physical quantity to be measured varies, with a known law of correspondence, which is possibly proportional.
Examples of physical quantities that can be measured with this type of solution are: level of liquids (fuel, oil, coolant), temperatures, and positions (engines, axles).
The circuit of FIG. 1 measures and converts into a first digital quantity D2 the voltage V2 across an unknown resistance RX (of unknown value).
Furthermore, the circuit measures and converts into a second digital quantity D1 the voltage V1 across a reference resistance of a known value RREF.
The circuit is arranged for calculating the digital representation DX of the value of the unknown resistance RX, in particular as ratio between the two voltages.
With reference to FIG. 1, the two resistances RREF and RX are set in series and connected between the supply voltage VSUP and ground gnd. The current IX that flows in the unknown resistance RX is
      I    X    =                    V        1                    R        REF              .  
Consequently, the value of the unknown resistance RX can be calculated, as a function of the quantities identified in the circuit, as:
      R    X    =                    V        2                    I        X              =                            R          REF                ⁢                              V            2                                V            1                              =                                    R            REF                    ⁢                                    D              2                                      D              1                                      =                              R            REF                    ⁢                      D            X                              
where RREF is the value of the known resistance, and DX is the digital representation of the value of the resistance RX.
The unknown resistance RX may, for example, be the sensitive element of a generic sensor device, such as for example a temperature sensor or a liquid-level sensor (for fuel or oil).
The two voltages V1 and V2 measured across the two resistances RREF and RX are supplied to two analog-to-digital converters ADC1 and ADC2. In alternative embodiments, the two analog-to-digital converters ADC1 and ADC2 can be implemented as a single converter ADC, in which the input is multiplexed between the input voltages V1 and V2.
To obtain the result sought, for example it is possible to calculate the ratio between the two voltages across the two resistances considered. In particular, the output of the second A/D converter may be divided by the output of the first A/S converter to supply a value that corresponds to the ratio between the two quantities and that is proportional to the unknown resistance.
The digital representation DX of the value of the unknown resistance RX is typically further processed, for example with filtering steps.
The known value of the resistance RREF is considered a scale factor for the digital representation DX of the value of the unknown resistance RX. Consequently, the operation of multiplication RREFDX is practically never carried out.
The background art described above corresponds to what is commonly presented in the specifications of the components that contain the function of interface/management of the resistive sensors.